1. Field of the Invention
The present invention relates to a method for filtrating and producing a polymer solution, and a method for preparing a solvent.
2. Description Related to the Prior Art
A cellulose acylate film is used as a film base of a photosensitive material since it has strength and flame resistance. In the cellulose acylate film, especially preferable one to the use of the film base is a cellulose triacetate film (hereinafter TAC film) formed of cellulose triacetate having acetyl value from 57.5% to 62.5%. Further, the TAC film is excellent in optical isotropy. Therefore it is adequate to use the TAC film as a color filter, an optical compensation film and a protective film for a polarizing filter in a liquid crystal display whose market becomes larger recently.
The TAC film is produced in a solution casting method. In the solution casting method. A film of the solution casting method is more excellent in optical characters and physical properties than that by other method, such as the melt casting method. In the solution casting method, a polymer is dissolved to a solvent (mainly organic solvent) to produce a polymer solution (hereinafter dope), and the dope is cast on a band or a drum to form the polymer film. The produced polymer film is used as a film base of a photo film or an optical film.
A mixture solvent is often used for preparing the dope. The main solvent of the mixture solvent is usually dichloromethane (called methylene chloride) which is a good solvent of TAC. Further, the mixture solvent contains alcohols, ketones and the like which are poor solvents of TAC. Further, additives are added to the dope in order to obtain the polymer film having a predetermined physical property. The additives are, for example, triphenylphosphate (TPP) and biphenyldiphenylphosphate (BDP). And the mixture solvent usually contains water. The water is originally contained in raw materials of TAC (for example a cotton linter and the like), alcohols or ketones (to which water easily dissolves), or the like. While the dope is fed or processed, the water in the air migrates into the dope.
In order to obtain the polymer film from the dope, the dope is cast on a base. Then, the drying of the dope and the polymer film is made to evaporate a solvent vapor, and the solvent vapor is recovered from the air as a recovery solvent. The recovery solvent is reused for the mixture solvent for preparing the dope. In order to recover to the solvent vapor, there are a condensing method and an absorbing method. The solvent vapor which is evaporated in the drying of the dope and the film is recovered at first in the condensing method, and thereafter the solvent vapor in which is evaporated in a heat-drying process is recovered in the absorb-recovery method, while the heat-drying process is made in a situation of little content of the solvent in the film. In the absorbing method, the solvent vapor is adsorbed to an adsorptive, and then a desorption gas is applied to the adsorptive to remove from the adsorptive the solvent vapor (for example dichloromethane), TAC and decomposition thereof, plasticizer and decomposition thereof, and the like. Thereby, hydrochloric acid is generated from dichloromethane and decompositions thereof. Acetic acid is generated from TAC and decomposition thereof, and phosphoric acid is generated from TPP, BDP and decomposition thereof. As hydrochloric acid and phosphoric acid cause the oxidization of inner surfaces of pipes and tanks, an alkali solution is added to the recovery solvent (or the mixture solvent) so as to neutralize the recovery solvent as described in Japan Institute of Invention and Innovation (JIII) JOURNAL of TECHNICAL DISCLOSURE 2001-1745.
In the dope, there are types of polymers whose raw materials are natural materials. The raw material contains a small amount of substances which are not or hardly dissolved to the solvent. Further, the dope often contains foreign substances or dusts. Originally the former is contained in polymer or other raw materials and the latter is mixed in feeding process or dissolving process of the raw material. Note that undissolved substances are determined in the following description as a general word of an undissolved polymer remaining in the solvent, a polymer in sol-like state in the dope, impurities in the raw materials, and the foreign substances. Usually, the dope contains at least two different sizes of the undissolved substances. The undissolved substances are removed with a filter medium after preparing and before casting the dope to reduce that the produced film has defects. Japanese Patent Laid-Open Publication No. 2000-256477 discloses a method in which a filter medium having micro pore is used for removing the undissolved substances from the dope.
Further, in order to remove all of the undissolved substances, the filter medium has pore smaller than the minimum size of the undissolved substance. In these cases, undissolved substances occlude pore of the filter medium and stop it. Accordingly, the filtering life becomes extremely short. Further, there is sometimes a case in which the filtration pressure applied for filtrating the dope becomes higher in the stop of the pore, which causes the interruption of producing the dope in the dope production line.
However, the dope may contain micro undissolved substances enough to form the excellent film in optical properties and strength and so on. Accordingly, a standard size is determined as a limit of the size of the undissolved substance, in which it has no bad influence on the formed film. In FIGS. 14A-14C, a filter medium 310 for filtrating a dope 312 is formed of cellulose fiber and has a hole 311 of diameter smaller than the standard size. The pore 311 are formed in a feeding direction of the dope 312.
The inventor considers about the phenomena, as shown in FIGS. 14A-14C. FIGS. 14A-14C are temporary situation of the pores of the filter medium. Accordingly, the situations of atomic groups, molecular and ions are not restricted in these figures. In FIG. 14A, there are hydroxyl groups 313, 317 and carboxyl groups 314, 318 in the filter medium 310. Some of carboxyl groups 314 are ionized to generate a hydrogen ion 314a and carboxyl anion 314b. Note that the ratio of the number of the carboxyl group 314 to the hydroxyl group 313 exposed on the pore wall 310a is larger in this figure than in real.
TAC is produced by performing the acetylation of the hydroxyl group. However, as the material of cellulose is natural product, the TAC pellet contains calucium carboxylate, magnesium carboxylate and their derivatives, which form the undissolved substances 315 when the TAC pellet is dissolved to the solvent. Note that the group R in the undissolved substance 315 may be several groups, such as alkyl chain, other functional groups, groups in which substitution of some atoms of the alkyl chain is made. The length of the group R in the undissolvend particle 315 is at least about that of oligomer, as the impurities is originally from the natural materials. And the length of the large undissolved substances may be about that of polymer. Further, the group X is atomic group, such as alkyl chain, functional group, and the like.
As shown in FIG. 14B, the undissolved substance 315 often dissociates to a calcium carboxylate cation 315a and an anion 315b. The calcium carboxylate cation 315a and carboxylate anion 314b forms an ionic bond 316a. 
Further, first type hydrogen bonds 317a are formed between the oxygen of C═O bond in the calcium carboxylate cation 315a and the hydroxyl group 317 on the pore wall of the pore 311. The hydrogen bond is originally weak. However, as there are a quite large number of hydroxyl groups on the pore wall, the calcium carboxylate is bound to the pore wall with strong force.
Furthermore, second type hydrogen bonds 318a are formed between oxygen of C═O bond in the calcium carboxylate cation 315a and the hydrogen of the carboxyl group 318. The number of the second type hydrogen bonds 318 is very small. Accordingly, the second type hydrogen bonds 318 hardly causes the adhesion of the foreign substances to the pore wall. Therefore, the effects of the second type hydrogen bonds 318 to the stop of the pore is quite small.
The ionic bond is stronger than the hydrogen bond since being formed with a stronger electric force. Accordingly, the ionic bond 316 may cause the stop of the pore. Further, as the number of the first hydrogen bonds 316a is very large and the total force thereof becomes large. Accordingly, the first hydrogen bond 316a may cause the stop of the pore.
Further, undissolved substances often have a property to form a agglomerate or an association. In this case, the pore is stopped with the agglomerate or the association of the undissolved substances.
In order to neutralize the recovery solvent, sodium hydroxide solution is used since sodium hydroxide is cheap. In this case, however, brown impurities are generated in the dope while the recovery and the neutralization of the mixture solvent is made several times. Sometimes the dope containing the brown impurities is cast on the base, and becomes brown or black foreign material in the produced polymer film. Although other alkali solution is used, this problem cannot be solved, and the effects of production of the polymer film don't become higher. Further, impurities are generated in accordance with hydrogen ion concentration of the mixture solvent (organic solvent) and the dope that contain dichloromethane insoluble to water.
However, the pH value of aqueous solution is measured in a measuring method of the pH value, but that of organic solvent is not measured. Accordingly, neutralization agents are added to the mixture solvent on the basis of the amount the acidic or basic materials. However, in this case, a predetermined amount of the mixture solvent is supplied in a tank for performing the neutralization of the mixture solvent in a batch manner. Accordingly, the hydrogen ion concentration of the mixture solvent cannot be continuously regulated, which reduces the continuous supply of the dope in a film production line.